Parvoviridae
Parvoviridae is a family of the smallest known mammalian viruses, characterized by their linear single-stranded genome and an icosahedral protein capsid. These viruses, approximately 25 to 30 nanometers in diameter, are widespread in both birds and mammals, including humans, dogs, and cats. Notably, parvoviruses can cause severe diseases in animals, such as feline panleukopenia and canine parvovirus, but they are species-specific, meaning human strains do not pose a risk to other animals.
In humans, the most common parvovirus, B19, is responsible for erythema infectiosum, or fifth disease, predominantly affecting children. The virus is transmitted through respiratory droplets and contact with infected secretions, leading to symptoms like a characteristic rash and flu-like manifestations. While healthy individuals generally experience mild symptoms, those with compromised immune systems or specific blood disorders may face serious health risks, including severe anemia. There are no vaccines for human parvoviruses, and treatment is typically symptomatic, although antibody serum may be used for immunocompromised patients. The complexity of this virus family highlights both its clinical significance and the ongoing need for research into its interactions with other viruses and host organisms.
Parvoviridae
- TRANSMISSION ROUTE: Blood, inhalation
Definition
The smallest known mammalian viruses, the Parvoviridae family (parvoviruses) viruses, each has a linear single-stranded genome that encodes approximately two to three proteins enclosed within an icosahedral protein capsid.
Natural Habitat and Features
Parvoviruses are widespread in nature and are found in birds and in mammals such as humans, dogs, cats, and rodents. Parvovirus infection of cats (feline panleukopenia virus) and dogs (canine parvovirus) can result in potentially life-threatening disease for the animals; vaccines exist for the protection of cats and dogs. Despite the severity of infection in nonhuman animals, the parvoviruses are species-specific, so humans are at no known risk from those strains found in other animals.
Human parvoviruses, however, cause the common childhood disease erythema infectiosum. The reservoir for the human virus is unknown. Infection is so common that it is likely passed easily among susceptible children. Human transmission occurs through coughing, sneezing, and contact with infected saliva or mucus. The adeno-associated parvoviruses are defective and, as the name indicates, require the presence of respiratory adenoviruses or other helper viruses to replicate.
Pathogenicity and Clinical Significance
The most common strain of human parvovirus is B19, the etiological agent for erythema infectiosum, which is more commonly known as fifth disease because it was historically the fifth rashlike illness of childhood. Although the virus is associated with a variety of diseases, serious illness is found primarily in persons with compromised immune systems. Fifth disease is manifested as a rash displaying a slapped-face appearance, which generally lasts no more than several days. The rash may spread to the infected person’s trunk after several days. The virus targets replicating red blood cells in the bone marrow, resulting in the death of red cell progenitors. Inhibition of red cell production for release into the circulation may persist for as long as one week.
Because the life span of circulating erythrocytes is several months, the temporary suppression of cell replacement should pose no problem in an otherwise healthy person. However, the suppression of red cell production can result in severe anemia in immunocompromised persons. In addition to the characteristic rash, persons with fifth disease generally exhibit flulike symptoms such as fever, aching in joints, and chills. A minimum of two additional viral strains (K71 and V9) similar to B19 are known, although the clinical importance of these strains is unknown. Infection in adults by B19 may be more severe, with inflammation of joints resembling that in rheumatoid arthritis. Symptoms may persist for years.
B19 infection in pregnant women during the first trimester is rare, but an infection may result in fetal death (hydrops fetalis). Parvoviruses may also cause risks to pregnant women, including damage to the fetus, fetal anemia, and miscarriage. Infection of persons with red blood cell abnormalities, such as those with sickle cell anemia, thalassemia, or other forms of anemia, may become severe because red cell depletion is a result of viral infection in the bone marrow.
Another type of parvovirus included in the genus Bocaparvovirus, a subfamily of Parvovirinae, was isolated from children with lower respiratory infections. Subsequently, this Bocavirus was also found to play a role in coinfections and found to cause a wider array of more severe illnesses.
Members of the genus Dependovirus are replication defective and require the presence of a helper virus for replication. Members are designated as adeno-associated viruses (AAVs) because, historically, the adenoviruses, etiological agents of respiratory illnesses, were the first-known helper viruses for AAVs. However, other viruses, including the papillomaviruses (warts) and the herpesviruses, have also been shown to provide helper functions for AAVs. The actual helper function provided by these viruses needs to be clarified, and it may involve the activation of cell functions required by AAVs. The AAVs are known to integrate into the host chromosome during these infections. The clinical significance of this is unclear. The widespread presence of antibodies against AAVs among adults suggests infection by these viruses is common.
Ironically, coinfection of cervical cells by AAVs with strains of papillomavirus that are associated with the development of cervical cancer may actually reduce the chances of cancer development. The evidence is indirect. Replication of papillomaviruses is partially suppressed under these conditions of coinfection, although women with cervical cancer show minimal production of antibodies against AAVs.
Drug Susceptibility
Parvovirus infections commonly occur in children and are generally not medically significant in persons with standard immune systems. Conditions are treated symptomatically. More severe illnesses in persons with blood cell deficiencies, such as sickle cell anemia or thalassemia, may require replacement transfusions.
Immunocompromised persons may be treated with a serum containing antibodies directed against parvoviruses such as B19. Still, they may be at risk for chronic parvovirus infection. Parvoviruses other than B19 are usually asymptotic and cause much milder infections. This treatment would provide a form of short-lived passive immunity. No vaccines exist for the human strains of parvoviruses.
Bibliography
"About Parvovirus B19 - Parvovirus B19 and Fifth Disease." CDC, 13 Aug. 2024, www.cdc.gov/parvovirus-b19/about/index.html. Accessed 12 Oct. 2024.
Allander, Tobias, et al. "Human Bocavirus and Acute Wheezing in Children." Clinical Infectious Diseases, vol. 44, no. 7, 2007, pp. 904-910, doi.org/10.1086/512196. Accessed 12 Oct. 2024.
Brooks, George, et al. Jawetz, Melnick, and Adelberg’s Medical Microbiology. 25th ed., New York: McGraw-Hill, 2010.
Kerr, Jonathon, et al., editors. Parvoviruses. New York: Oxford University Press, 2006.
Kerr, J. R., et al. "Successful Intravenous Immunoglobulin Therapy in 3 Cases of Parvovirus B19-Associated Chronic Fatigue Syndrome." Clinical Infectious Diseases, vol. 36, no. 9, 2003, pp. e100-e106, doi.org/10.1086/374666. Accessed 12 Oct. 2024.
Strauss, James, and Ellen Strauss. Viruses and Human Disease. 2nd ed., New York: Academic Press/Elsevier, 2008.
Wagner, Edward, and Martinez J. Hewlett. Basic Virology. 3rd ed., Malden, Mass.: Blackwell Science, 2008.